In recent years, a turbocharger that is greatly effective to improvement of fuel efficiency is widely used in an automotive engine, particularly an automotive diesel engine, in strongly demanding improvement of fuel efficiency.
Turbocharger rotates a turbine wheel utilizing an exhaust gas from an engine to drive a compressor wheel provided on the same shaft, thereby supplying high pressure air to the engine.
FIG. 3A shows a structure of a general automotive turbocharger.
As shown in FIG. 3A, a turbocharger 10 has a turbine wheel 14 in a turbine housing 12 and a compressor wheel 18 in a compressor housing 16, and the turbine wheel 14 and the compressor wheel 18 are integrally connected in a rotational state by a common rotor shaft 20.
In the turbocharger 10, an exhaust gas from an engine is allowed to flow into the turbine housing 12 to rotate the turbine wheel 14 by the exhaust gas, thereby integrally rotating the compressor wheel 18 in the compressor housing 16.
Air is sucked into the compressor housing 16 by rotation of the compressor wheel 18 and pressurized, and high pressure air is supercharged to an engine.
FIG. 3B shows a shape of the turbine wheel 14 in more detail.
As shown in the drawing, the turbine wheel 14 includes a shaft part 22 of a rotation center and a plurality of wing parts 24 radially projected from the shaft part 22, and thus has a complicated shape as a whole.
Furthermore, the thickness thereof differs between the shaft part 22 and the wing parts 24. That is, the thickness is large in the shaft part 22 of rotation center, and the thickness is small in the wing parts 24.
Additionally, even in the wing parts 24, the thickness differs in each site. That is, the thickness is large in a root portion near the shaft part 22, and the thickness becomes smaller towards a tip of the wing part 24.
In the case of the turbine wheel 14 of an automotive turbocharger, the thickness is 1 mm or less in the portion having the smallest thickness.
A turbine wheel that rotates by receiving exhaust from an engine rotates at high speed (for example, the number of revolution per minute is hundreds of thousands) under high temperature (for example, under high temperature of about 950° C.). Therefore, the turbine wheel is required to have high strength at high temperature.
For this reason, a Ni-based alloy having excellent high temperature strength, particularly a Ni-based casting alloy represented by INCONEL 713C (trade name of International Nickel Company) has conventionally been mainly used as a material of a turbine wheel.
In the case of a Ni-based alloy having excellent high temperature strength, γ′ phase (gamma prime phase) (phase of Ni3(Al,Ti,Nb) of an intermetallic compound) precipitated as a strengthening phase is stable up to high temperature. Therefore, it is difficult to produce a turbine wheel by forging, and generally, a turbine wheel is cast mainly using a Ni-based casting alloy and is used as it is (as-cast state).
A turbine wheel is used under severe conditions such as high speed rotation under high temperature and rapid change of the number of revolution, and is therefore required to have characteristics on strength. In addition to this, in the case of casting using a Ni-based casting alloy, it is first important that when casting, a melt arrives in every corner of a product (in every corner of molding cavities) without solidifying the melt in the course of casting, whereby a beautiful product shape can be formed, and blow holes are not formed in the inside of the product. Conventionally, it was actual situation that production conditions for mainly achieving those have been pursued.
On the other hand, it is viewed as problematic that regarding high temperature strength, deviation occurs among products, and to pursue its cause, observation of the state of carbide and crystal grain has been mainly made, but the solution has not been achieved. Thus, the problems have still remained on occurrence of deviation and difference in high temperature strength.
As the background art to the present invention, the invention regarding an “alloy for an anvil” is shown in Patent Document 1 mentioned below, and it discloses an alloy for an anvil having a composition including, in terms of % by weight, C: 0.008 to 0.3%, Si: 0.1 to 0.5%, Mn: 0.1 to 0.25%, Cr: 8.0 to 22.0%, Mo: 3.5 to 10.0%, Nb+Ta: 1.5 to 5.0% in total, Al: 5.0 to 6.50%, Ti: 0.5 to 3.0%, Zr: 0.05 to 0.15%, B: 0.005 to 0.015% with the remainder being Ni. However, Patent Document 1 does not contain the description that high temperature strength is enhanced by controlling a size of γ′ phase in each site of a product, and therefore differs from the present invention.
Patent Document 2 discloses the invention relating to a “heat resistant elastic machine element and method for producing the same”, and it is disclosed therein that a plate-shaped heat resistant elastic machine element is formed by precision casting (reduced pressure suction casting method using a lost wax mold) using a Ni-based super heat-resistant alloy material having given components.
Patent Document 3 shows the invention relating to a “nickel-based heat-resistant alloy”, and a Ni-based heat-resistant alloy prepared such that (Al,Cr)2O3 coating film is formed on the surface thereof by adding Al and Cr in combination is disclosed therein.
Patent Document 4 shows a “heat-resistant alloy”, and discloses a Ni-based heat-resistant alloy that enabled casting and molding of a complicated shape part having excellent high temperature creep characteristic by adding 0.20% or less of REM (Rare Earth Metal) in order to eliminate adverse influence of Se contained in a melting raw material to creep rupture strength.
However, those Patent Documents 2 to 4 do not contain the description that high temperature strength is enhanced by controlling a size of γ′ phase in each site of a product, and therefore differ from the present invention.
Patent Document 1: JP-A-1-255635
Patent Document 2: JP-A-6-41664
Patent Document 3: JP-A-4-358037
Patent Document 4: JP-A-60-258444